Biomolecules, such as proteins, are remarkable in their capability to self-assemble into well-defined and intricate structures. The most intriguing self-assembly process is the folding of peptide chains into native protein structures. The fundamental building blocks in proteins are not simple canonical secondary structures (such as .alpha.-helices and .beta.-sheets), but characteristic assemblies of secondary structural elements. Among the protein assemblies are the simple .beta./.alpha./.beta. motif, the hairpin, and the .alpha.-helical coiled-coil, as well as the more complicated four .alpha.-helical bundle, the doubly wound .beta.-sheet, the Jelly roll, and the Greek key.
Many researchers have attempted to create protein-like assemblies for the purpose of studying protein folding, and to create new biomaterials for use in medical devices and other medical applications such as drug delivery systems. The most common assembly used for protein design is the four .alpha.-helical bundle, which has been developed as a synthetic enzyme, for redox catalysis, for antibody production, as ion channels in lipid bilayers, and as surface mimetics of human class I MHC. The collagen-model triple-helix has also been used for protein design. Synthetic triple-helical proteins have incorporated native type IV collagen sequences that promote adhesion and spreading of tumor cells and native type III or IV collagen sequences that induce the aggregation of platelets. See, for example, Fields et al., J. Biol. Chem., 268, 14153-14160 (1993); Miles et al., J. Biol. Chem., 269, 30939-30945 (1994); Grab et al., J. Biol. Chem., 271, 12234-12240 (1995); Morton et al., Thrombosis Res., 72, 367-372 (1993); and Rao et al., J. Biol. Chem., 269, 13899-13903 (1994).
The triple-helix is a super-secondary structure characteristic of collagen. Collagen-like triple-helices are also found in macrophage scavenger receptors types I and II and bacteria-binding receptor MARCO, complement component Clq, pulmonary surfactant apoprotein, acetylcholinesterase, and mannose binding protein. The triple-helix consists of three polypeptide chains, each in an extended, left-handed polyPro II-like helix, which are staggered by one residue and then supercoiled along a common axis in a right-handed manner. Geometric constraints of the triple-helical structure require that every third amino acid is Gly, resulting in a Gly-X-Y repeating sequence. Stability of the triple-helix depends upon the imino acid content. Furthermore, hydroxyproline (Hyp) stabilizes the triple-helical structure by facilitating the formation of a hydrogen bonding network with surrounding water molecules. For simple collagen-model peptides, (Gly-Pro-Hyp).sub.n forms the most thermally stable triple-helices, with a melting temperature (T.sub.m) of 58-60.degree. C. when n=10 (SEQ. ID NO:3).
Several strategies have been employed in order to induce triple-helical structure formation in isolated collagen ligand sequences. See, for example, Fields, Connect. Tissue Res., 31, 235-243 (1995). Simply adding a number of Gly-Pro-Hyp repeats to both ends of a collagenous sequence can, under certain circumstances, induce triple-helical conformation. However, even with more than 50% of the peptide sequence consisting of Gly-Pro-Hyp repeats, the resulting triple-helices still may not have sufficient thermal stability (T.sub.m &lt;37.degree. C.) to survive physiological conditions. Substantial stabilization of the triple-helical structure can be achieved with the introduction of covalent links between the C-terminal regions of the three peptide chains. See, for example, Fields et al., J. Biol. Chem., 268, 14153-14160 (1993); Grab et al., J. Biol. Chem., 271, 12234-12240 (1995); Fields et al., Biopolymers, 33, 1695-1707 (1993); Fields et al., Lett. Peptide Sci., 3, 3-16 (1996); and Fields et al., Anal. Biochem., 231, 57-64 (1995). However, the large size (90-125 amino acid residues) of the resulting "branched" triple-helical peptide compounds make them difficult to synthesize and purify. Ideally, one would like to create a system by which synthetic linear peptide chains self-assemble into desirable secondary structures (including super-secondary structures).
Thus, what is still needed are complexes of synthetic linear peptide chains that self-assemble into secondary structures. Specifically, what is needed are approaches to building a collagen-like structural motif that facilitate peptide alignment and structure initiation and propagation.